Cell search and measurement in heterogeneous networks

ABSTRACT

An extended cell search procedure enables more inclusive measurement reports by mobile terminals operating in a heterogeneous network. The mobile terminal may be configured to conduct an extended cell search to enable better detection of signals transmitted from weaker cells. For mobile terminal with extended cell search capabilities, the network sends an extended cell search message to the mobile terminal when there is a need for an extended cell search. In response to the extended cell search message, the mobile terminal uses an extended cell search procedure rather than the normal cell search procedure (as specified in Rel-8 of the LTE standard) when performing cell searches.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/899,780, filed Oct. 7, 2010, which claims the benefit of U.S.Provisional Patent Application 61/354,900, filed Jun. 15, 2010, titled“Cell Search and Measurements in Heterogeneous Network,” which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to heterogeneous networkscomprising a mixture of low power and high power nodes with overlappingcoverage and, more particularly, to cell searching and measurement in aheterogeneous network.

BACKGROUND

The new Third Generation Partnership Project (3GPP) standard known asLong Term Evolution (LTE) (3GPP LTE Rel-10) supports heterogeneousnetworks. In heterogeneous networks, a mixture of cells of differentsize and overlapping coverage areas are deployed. For example, aheterogeneous network may deploy pico cells served by relatively lowpower nodes within the coverage area of a macro cell served byrelatively high power base stations. Heterogeneous networks could alsodeploy relatively low-power home base stations and relays to provideimproved service in indoor areas. The aim of deploying low power nodes,such as pico base stations, home base stations, and relays, within amacro cell where coverage is provided by a high power base station, isto improve system capacity by means of cell splitting gains as well asto provide users with wide area experience of very high speed dataaccess throughout the network. Heterogeneous deployment schemesrepresent one alternative to deployment of denser networks of macrocells and are particularly effective to cover traffic hotspots, i.e.small geographical areas with high user densities served by lower powernodes.

In heterogeneous networks, there may be a large disparity in outputpower of the low power nodes compared to the base stations serving macrocells. For example, the output power of the base stations in the macrocells may be in the order of 46 dBm while the output power of the lowpower nodes in the pico cells may be less than 30 dBm. In Release 8 and9 of the LTE standard, a mobile terminal is required to find and performsignal measurements for cells with a Es/Iot ratio greater than or equalto −6 dB. Thus, there is no requirement that the mobile terminal findsand reports pico cells in some heterogeneous deployment scenarios.Further, the large difference in output power of the macro cellscompared to the pico cells may make measurements of signals transmittedby the pico cells difficult.

Therefore there is a need for a method and apparatus enabling mobileterminals operating in heterogeneous networks detect and measure signaltransmitted by pico cells that could be up to 10-15 dB below the signalstransmitted by overlapping macro cell base stations.

SUMMARY

An extended cell search procedure is described to enable more inclusivemeasurement reports by mobile terminals in a heterogeneous network. Themobile terminal may be configured to conduct an extended cell search toenable better detection of signals transmitted from weaker cells. Formobile terminal with extended cell search capabilities, the networksends an extended cell search message to the mobile terminal when thereis a need for an extended cell search. The need for an extended cellsearch may arise, for example, when the mobile terminal is operating inor near an area served by both pico cells and macro cells. When picocells or other low power access nodes are present in the generalvicinity of the mobile terminal, the network can instruct the mobileterminal to use an extended cell search procedure. In response to theextended cell search message, the mobile terminal uses an extended cellsearch procedure rather than the normal cell search procedure (asspecified in Rel-8 of the LTE standard) when performing cell searches.

Accordingly, exemplary embodiments of the present invention compriseextended cell search procedures implemented by a base station in aheterogeneous network. The base station determines whether a mobileterminal served by the base station has extended cell search capability.If so, the base station sends an extended cell search message to themobile terminal to enable extended cell searching by the mobileterminal. The base station may also send a transmitting a measurementmap to the mobile terminal relating cell identities of candidate cellsand corresponding subframes for performing measurements.

Other embodiments of the invention comprise a base station configured tosupport extended cell searches. In one exemplary embodiment, the basestation comprises a transceiver for communicating with mobile terminalsand a control circuit to control operation of the transceiver. Thecontrol circuit is configured to determine whether a mobile terminalserved by the base station has extend cell search capability and to sendan extended sell search message to the mobile terminal when an extendedcell search is needed. The control circuit may be further configured tosend a measurement map to the mobile terminal relating cell identitiesof candidate cells and corresponding subframes for performingmeasurements.

Other exemplary embodiments of the present invention comprise extendedcell search procedure implemented by a mobile terminal in aheterogeneous network. In one exemplary embodiment, the mobile terminalreceives an extended cell search message and a measurement map relatingcell identities of candidate cells and corresponding subframes forperforming measurements. The mobile terminal, in response to theextended cell search message begins performing extended cell searchesbased on the received measurement map.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a heterogeneous communication networkaccording to the present invention.

FIG. 2 illustrates a link imbalance scenario.

FIG. 3 illustrates an exemplary extended cell search procedureimplemented by a base station or other network node.

FIG. 4 illustrates an exemplary extended cell search procedureimplemented by an active mobile terminal.

FIG. 5 illustrates an exemplary extended cell search procedureimplemented by a mobile terminal during power-up.

FIG. 6 illustrates an exemplary mobile terminal with extended cellsearch capability.

FIG. 7 illustrates an exemplary base station for a communication networksupporting extended cell searches.

DETAILED DESCRIPTION

Turning now to the drawings, FIG. 1 illustrates an exemplaryheterogeneous communication network 10 according to one exemplaryembodiment of the present invention. The present invention is describedin the context of a Long-Term Evolution (LTE) network, which isspecified in Release 10 of the LTE standard. However, those skilled inthe art will appreciate that the invention may be applied inheterogeneous networks using other communication standards.

The communication network 10 comprises a plurality of macro basestations or other high power access nodes 200 providing radio coveragein respective macro cells 20 of the communication network 10. In theexemplary embodiment shown in FIG. 1, three pico cells 30 served by lowpower access nodes 300 are deployed within the macro cell 20. The lowpower access nodes may comprise pico base stations or home basestations. The output power of the high power access nodes is presumed tobe in the order of 46 dBm, while the output power of the low poweraccess nodes is presumed to be less than 30 dBm.

In some heterogeneous networks 10, frequency separation between thedifferent layers, i.e. macro and pico cells 20, 30 in FIG. 1, is used toavoid interference between the high power and low power access nodes 200and 300 respectively. When frequency separation is used, the high poweraccess nodes 200 and low power access nodes 300 operate on differentnon-overlapping carrier frequencies to reduce interference between themacro and pico layers. Cell splitting gains are obtained when the radioresources in the pico cell 30 can be simultaneously used when the macrocell 20 is transmitting. The is that drawback of frequency separationmay lead to resource-utilization inefficiency. For example, when thepico cell 30 is lightly loaded so that its resources are not fullyutilized, it may be more efficient to assign all carrier frequencies tothe macro cell 20. However, the split of carrier frequencies betweenlayers is typically static.

In some heterogeneous networks 10, radio resources on same carrierfrequencies are shared by the macro and pico layers by coordinatingtransmissions in the overlapping macro and pico cells 20, 30. This typeof coordination is referred to as inter-cell interference coordination(ICIC). Certain radio resources are allocated for the macro cells 20during some time period whereas the remaining resources can be accessedby pico cells 30 without interference from the macro cell 20. Dependingon the load distribution across the layers, the resource split canchange over time to accommodate different load distributions. Incontrast to the splitting of carrier frequencies, sharing radioresources across layers using some form of ICIC can be made more or lessdynamic depending on the implementation of the interface between theaccess nodes. In LTE, an X2 interface has been specified in order toexchange different types of information between access nodes. Oneexample of such information exchange is that a base station 200 caninform other base stations 200 that it will reduce its transmit power oncertain resources.

In order to establish a connection with the LTE network 10, the mobileterminal 100 needs to find and acquire synchronization with a cell 20,30 within the network 10, read system parameters from a broadcastchannel in the selected cell 20, 30, and perform a random accessprocedure to establish a connection with the selected cell 20, 30. Thefirst of these steps is commonly referred to as cell search. To assistthe mobile terminal 100 in the cell search procedure, the base station200 transmits two synchronization signals on the downlink; the PrimarySynchronization Signal (PSS) and the Secondary Synchronization Signal(SSS). The synchronization signals transmitted in each cell 20, 30comprise a specific set of sequences that define the cell identity. TheLTE standard specifies the location in time and frequency of thesynchronization signals. Thus, by detecting the synchronization signals,the mobile terminal 100 will acquire the timing of a candidate cell 20,300, and, by observing which of multiple sequences the cell istransmitting, the mobile terminal 100 can identity of the cell 20, 30.

A mobile terminal 100 does not carry out cell search only at power-up,i.e. when initially accessing the system. In order to support mobility,the mobile terminals 100 need to continuously search for, synchronizeto, and estimate the reception quality of signals transmitted byneighbor cells. The mobile terminals 100 may evaluate the receptionquality of signals from the neighbor cells, in comparison to thereception quality of the current serving cell, to determine whether ahandover (for mobile terminals 100 in connected mode) or cellre-selection (for mobile terminals 100 in idle mode) should be carriedout. For mobile terminals 100 in connected mode, the network 10 makesthe handover decision based on measurement reports provided by themobile terminals 100.

The measurement reports provided by the mobile terminal 100 may includemeasurements of the reference signal received power (RSRP) and/orreference signal received quality (RSRQ). Depending on how thesemeasurements, possibly complemented by a configurable offset, are used,the mobile terminal 100 can be connected to the cell 20, 30 with thestrongest received power, or the cell 20, 30 with the lowest path loss,or a combination of the two. These selection criteria (received powerand path loss) do not necessarily result in the same selected cell 20,300. Because the output power varies for different types of access nodes200, 300, it is possible that, for a given mobile terminal 100, theaccess node 200, 300 with the highest RSRP and RSRQ measurements and theaccess node 200, 300 with the lowest path loss are different. Thissituation is referred to herein as link imbalance.

FIG. 2 illustrates how link imbalance can occur in a heterogeneousnetwork 10. It is realistically presumed for purposes of this examplethat the output power of a pico base station 300 in the pico cell 30 isin the order of 30 dBm or less, while the output power of the macro basestation 200 is in the order of 46 dBm. Consequently, when the mobileterminal 100 is operating near the cell edge of the pico cell 30, thereceived signal strength from the macro cell 20 can be much larger thanthat of the pico cell 30. However, the path loss to the base station 200in the macro cell 20 may be greater than the path loss to the pico basestation 300 in the pico cell 30. In FIG. 3 the downlink border indicatesthe point at which the received signal strength from the macro cell 20and pico cell 30 is equal. The uplink border indicates the point atwhich the path loss to the base stations 200, 300 in the macro cell 20and pico cell 30 respectively are equal. The region between the DL andUL borders is the link imbalance zone. From a downlink perspective, itmay be better for a mobile terminal 100 in the link imbalance zone toselect a cell 20, 30 based on downlink received power, but from anuplink perspective, it may be better to select a cell 20, 30 based onthe path loss because the transmit power of the mobile terminal 100 islimited. In this scenario, it might be preferable from a systemperspective for the mobile terminal 100 to connect to the pico cell 30even if the macro downlink is up to 10-20 dB stronger than the pico celldownlink. Therefore, the mobile terminal 100 needs to detect and measuresignals from the pico cells 30 even when there is a large disparity inthe RSRP and RSRQ measurements.

To enable more inclusive measurement reports, the mobile terminal 100can be configured to conduct an extended cell search to enable betterdetection of signals transmitted from the pico cells 30. For mobileterminal 100 with extended cell search capabilities, the network 10sends an extended cell search message to the mobile terminal 100 whenthere is a need for an extended cell search. The need for an extendedcell search may arise, for example, when the mobile terminal 100 isoperating in or near an area served by both pico cells 30 and macrocells 20. When pico cells 30 or other low power access nodes are presentin the general vicinity of the mobile terminal 100, the network 10 caninstruct the mobile terminal 100 to use an extended cell searchprocedure. In response to the extended cell search message, the mobileterminal 100 uses an extended cell search procedure rather than thenormal cell search procedure (as specified in Rel-8 of the LTE standard)when performing cell searches. When the extended cell search procedureis invoked, the mobile terminal 100 performs and reports measurementsfor cells 20, 30 where the received signal strength is lower thanspecified for Rel-8 mobile terminals 100. Thus, when an extended cellsearch is performed, the mobile terminal 100 may use a lower thresholdthan currently specified for Rel-8 mobile terminals 100 for signalmeasurement and reporting purposes.

The mobile terminal 100 may extend the cell search to detect signalsfrom the weaker cells 20, 30 in several ways. In some embodiments, theextended cell search may rely on the same Primary Sync Signal (PSS) andSecondary Sync Signal (SSS) for the standard cell search, but processthe signals differently to improve search performance. For example, themobile terminal 100 may use a longer (compared to Rel-8) averagingperiod to average the information contained in the PSS and/or SSS priorto the detection of the cell identity (cell ID). As another example, themobile terminal 100 may use more receive antennas in the extended cellsearch along with some form of combining, such as maximal ratiocombining (MRC). In other embodiments, the mobile terminal 100 cansearch for an expanded set of synchronization signals including the PSS,SSS, and other synchronization signals which may be specified forextended cell searches.

FIG. 3 illustrates an exemplary network procedure 400 implemented by abase station 200 in a serving macro cell 20. The base station 200determines whether the mobile terminal 100 has extended cell searchcapability (block 410). Typically, this determination is made by at thetime the mobile terminal 100 establishes a connection with the basestation 200. To briefly summarize, the base stations 200 monitors theRandom Access Channel (RACH) in order to detect mobile terminals 100attempting to connect to the base station 200. Once a new mobileterminal 100 is detected, further signaling between the base station 200and mobile terminal 100 may be carried out. For example, a mobileterminal 100 in idle mode may attempt to camp on the detected cell ormay perform a location update procedure. An active mobile terminal 100may send a connection request in order to establish a connection withthe cell. In any event, during the signaling procedure, the mobileterminal category/capability may be sent to the base station 200. Insome instances, the base station 200 may receive information about themobile terminal capability from another base station 200 or othernetwork node.

When there is a need for an extended cell search, the base station 200sends an extended cell search message to the mobile terminal 100 (block420). Whether an extended cell search is needed may depend on thedeployment scenario and/or the location of the mobile terminal 100. Ingeneral, an extended cell search may be needed when the mobile terminal100 is in or moving toward an area served by one or more pico cells 30.

The extended cell search message can be transmitted as a radio resourcecontrol (RRC) message. Alternatively, layer 1 signaling may be used tosend the extended cell search message. The extended cell search messageincludes, at least, an indication that one or more pico cells 30 servedby lower power nodes 300 are present in the vicinity of the mobileterminal 100 that could have signal-to-noise ratios (SNRs) below thereporting requirements of Release 8 LTE. The extended cell searchmessage could also include a neighbor list containing the physical cellidentities (PCIs) for the lower power access nodes 300 in the pico cells30. In some embodiments, the base station may send a measurement maprelating the PCIs to corresponding subframes for performing signalmeasurements. Thus, once a mobile terminal 100 has detected the PCI onthe neighbor cell list, the mobile terminal 100 will have informationabout the subframes, frequencies, or similar information, wheresynchronization signals may be expected. The measurement map may beincluded in the cell search message, or may be included in a separatemessage.

FIG. 4 illustrates an exemplary extended cell search procedure 500 for amobile terminal 100. The extended cell search procedure is initiatedwhen the mobile terminal 100 receives an extended cell search messagefrom the serving base station 200 (block 510). As previously noted, theextended cell search message includes an indication that there are picocells 30 in the vicinity of the mobile terminal 100 for which thereceived signal power may be lower than the minimum requirement forreporting as specified in Release 8 LTE. The extended cell searchmessage could also include a neighbor list including the PCIs of cells20, 30 to include in the extended cell search.

In some embodiments, the mobile terminal 100 also receives a measurementmap from the serving base station 200 relating the PCIs of the cells 20,30 in the neighbor cell list with corresponding subframes (block 520).In response to the extended cell search message, the mobile terminal 100periodically performs an extended cell search based on the receivedmeasurement map (block 530). For example, the mobile terminal may start,on a regular basis, performing signal strength measurements (RSRP/RSRQ)for cells identified by each PCI on the corresponding subframesindicated by the measurement map. As previously noted, the mobileterminal 100 may search for an expanded set of synchronization signalswhen performing the extended cell search. The type, time, and frequencypositions of the synchronization signals may be specified by standard,or may be communicated to the mobile terminal 100 in the extended cellsearch message, or other control signaling. The type, time, andfrequency of the additional synchronization signals could also bespecified in the measurement map transmitted by the base station 200. Inperforming the extended cell search, the mobile terminal 100 may employlonger averaging times or a larger number of antennas to increaselikelihood of detection of the synchronization signals.

FIG. 5 illustrates an alternative extended cell search procedure 600 fora mobile terminal 100. In this embodiment, the extended cell searchmessage is transmitted by the base station 200 on a broadcast channel.The extended cell search message may be included in a master informationblock (MIB) or secondary information block (SIB). When a mobile terminal100 with extended cell search capability is powered on, the mobileterminal searches for, and reads, the broadcast channel of a detectedcell 20, 30 in the vicinity of the mobile terminal 100 (block 610).Based on information transmitted on the broadcast channel, the mobileterminal 100 determines whether camping is allowed (block 620). If so,the mobile terminal begins camping on the detected cell according toconventional camping procedures (block 630). If camping is not allowed,the mobile terminal 100 detects whether an extended cell search messagewas broadcast (block 640). Camping may be disallowed, for example, ifthe detected cell 20, 30 belongs to a different service provider, or maybe disallowed based on user preferences. If the broadcast channel doesnot include an extended cell search message, the mobile terminal 100continues searching for other cells (block 650). If the broadcastchannel includes an extended cell search message, the mobile terminal100 performs an extended cell search as previously described to searchfor weaker cells (block 660).

FIG. 6 illustrates an exemplary mobile terminal 100 that implements theextended cell search procedure described herein. The mobile terminal 100comprises a transceiver 110, control circuit 120, and user interface130. The transceiver 110 comprises a standard cellular transceiveraccording to the LTE standard, or other standard now known or laterdeveloped, which supports extended cell search procedures. The controlcircuit 120 controls the operation of the mobile terminal 100 based oninstructions stored in memory (not shown). The control circuit 120 maybe implemented with one or more processors, hardware, firmware, or acombination thereof. The control circuit 120 is configured to implementthe procedure as shown in FIGS. 4 and 5. Persistent memory is used forstorage of program instructions. Program instructions to implement theextended cell search procedure may be stored in some form of persistentmemory (e.g., read-only memory). The control circuit may also includerandom access memory to store temporary data. The user interface 130typically comprises a display and one or more input devices to enablethe user to interact with and control the mobile terminal 100. The userinput devices may include a keypad, touchpad, function keys, scrollwheels, or other similar input devices. If the mobile terminal includesa touchscreen display, the touchscreen display may also function as auser input device.

FIG. 7 illustrates an exemplary base station for communicating with themobile terminal 100. The base station 200 comprises an antenna 210coupled to a transceiver 220, and a control circuit 230. The transceiver220 comprises a standard cellular transceiver operating according to theLTE standard, or other standard now known or later developed, supportingextended cell search procedures. The control circuit 230 controls theoperation of the base station 200. The functions performed by thecontrol circuit 230 include radio resource control and mobilitymanagement functions. The control circuit 230 may be implemented by oneor more processors, hardware, firmware, or a combination thereof. Thecontrol circuit 230 is configured to implement the procedure as shown inFIG. 3. Program instructions to implement the extended cell searchprocedure may be stored in some form of persistent memory (e.g.,read-only memory). The control circuit may also include random accessmemory to store temporary data.

The extended cell search procedure enables mobile terminals 100 to findcells for which the receive signal power is below the minimum levelspecified by the Release 8 LTE. Thus, measurement report sent by themobile terminal 100 to the base station 200 will be more inclusive and,thus, create opportunities to improve uplink coverage and systemcapacity.

The present invention may, of course, be carried out in other specificways than those herein set forth without departing from the scope andessential characteristics of the invention. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive, and all changes coming within the meaning and equivalencyrange of the appended claims are intended to be embraced therein.

What is claimed is:
 1. A method of cell searching implemented by amobile terminal, said method comprising: selecting between a normal cellsearch and an extended cell search based on control information receivedfrom a base station; if the normal cell search is selected, performingthe normal cell search using a first measurement and reportingthreshold; if the extended cell search is selected, performing theextended cell search using a second measurement and reporting thresholdlower than the first measurement and reporting threshold, wherein thefirst measurement and reporting threshold corresponds to a first Es/Iotratio between received power per resource element (Es) and receivedpower spectral density of noise and interference per resource element(Iot), and wherein the second measurement and reporting thresholdcorresponds to a second Es/Iot ratio.
 2. The method of claim 1 whereinperforming an extended cell search comprises: receiving synchronizationsignals from one or more candidate cells; and averaging informationcontained in said synchronization signals over a longer predeterminedperiod compared to the normal cell search.
 3. The method of claim 1wherein performing an extended cell search comprises receivingsynchronization signals from one or more candidate cells on an expandedset of receive antennas compared to the normal cell search.
 4. Themethod of claim 1 wherein performing an extended cell search comprisesreceiving and detecting an expanded set of synchronization signalscompared to the normal cell search from one or more candidate cells. 5.The method of claim 1, further comprising: receiving an extended cellsearch message; and receiving a measurement map relating cell identitiesof candidate cells and corresponding subframes for performingmeasurements, wherein the performing an extended cell search isresponsive to the extended cell search message based on the receivedmeasurement map.
 6. The method of claim 5 wherein receiving an extendedcell search message comprises receiving the extended cell search messagefrom a serving base station.
 7. The method of claim 6 wherein receivingan extended cell search message comprises receiving the extended cellsearch message in one of a radio resource control signaling message andlayer 1 signaling message.
 8. The method of claim 6 wherein themeasurement map is received in the extended cell search message.
 9. Themethod of claim 6 wherein the measurement map is received separatelyfrom the extended cell search message.
 10. The method of claim 6 whereinreceiving an extended cell search message comprises receiving theextended cell search message on a broadcast channel.
 11. The method ofclaim 5 wherein the extended cell search message includes an indicationof one or more candidate cells that the mobile terminal could select asa serving cell.
 12. The method of claim 5 further comprising performingsignal strength measurements for one or more of the candidate cellsidentified in the measurement map.
 13. The method of claim 1, whereinperforming the normal cell search using a first measurement andreporting threshold comprises measuring signals with an Es/Iot ratioequal or higher than the first Es/Iot ratio, which is −6 dB.
 14. Themethod of claim 1, wherein performing the extended cell search using asecond measurement and reporting threshold comprises measuring signalswith an Es/Iot ratio equal or higher than the second Es/Iot ratio, whichis less than −6 dB.
 15. The method of claim 1, wherein the Es/Iot ratiocomprises the Es/Iot ratio for synchronization signals.
 16. A mobileterminal comprising: a transceiver for communicating with a basestation; a control circuit to control operation of the transceiver, saidcontrol circuit configured to: select between a normal cell search andan extended cell search based on control information received from abase station; if the normal cell search is selected, perform the normalcell search using a first measurement and reporting threshold; if theextended cell search is selected, perform the extended cell search usinga second measurement and reporting threshold lower than the firstmeasurement and reporting threshold, wherein the first measurement andreporting threshold corresponds to a first Es/Iot ratio between receivedpower per resource element (Es) and received power spectral density ofnoise and interference per resource element (Iot), and wherein thesecond measurement and reporting threshold corresponds to a secondEs/Iot ratio.
 17. The mobile terminal of claim 16 wherein the controlcircuit is configured to perform an extended cell search by: receivingsynchronization signals from one or more candidate cells; and averagingsaid synchronization signals over a longer predetermined period comparedto the normal cell search.
 18. The mobile terminal of claim 16 whereinthe control circuit is configured to perform an extended cell search byreceiving synchronization signals from one or more candidate cells on anexpanded set of receive antennas compared to the normal cell search. 19.The mobile terminal of claim 16 wherein the control circuit isconfigured to perform an extended cell search by receiving an expandedset of synchronization signals compared to the normal cell search fromone or more candidate cells.
 20. The mobile terminal of claim 16,wherein said control circuit is configured to: receive an extended cellsearch message; and receive a measurement map relating cell identitiesof candidate cells and corresponding subframes for performingmeasurements, wherein the control circuit is configured to perform theextended cell search responsive to the extended cell search messagebased on the received measurement map.
 21. The mobile terminal of claim20 wherein the control circuit is configured to receive the extendedcell search message from a serving base station.
 22. The mobile terminalof claim 21 wherein the control circuit is configured to receive theextended cell search message in one of a radio resource controlsignaling message and layer 1 signaling message.
 23. The mobile terminalof claim 20 wherein the control circuit is configured to receive themeasurement map in the extended cell search message.
 24. The mobileterminal of claim 20 wherein the control circuit is configured toreceive the measurement map separately from the extended cell searchmessage.
 25. The mobile terminal of claim 20 wherein the control circuitis configured to receive the extended cell search message on a broadcastchannel.
 26. The mobile terminal of claim 20 wherein the extended cellsearch message includes an indication of one or more candidate cellsthat the mobile terminal could select as a serving cell.
 27. The mobileterminal of claim 20 wherein the control circuit is further configuredto perform signal strength measurements for one or more of the candidatecells identified in the measurement map.